An conventional electronic control unit having an abnormality diagnostic function of a soak timer includes a control portion. When an ignition switch of a vehicle is turned on, the control portion in the electronic control unit is supplied with power and executes various controls. After that, when the ignition switch is turned off and the power supply to the control portion is stopped at a certain time point (hereafter, referred to as an off-time of the ignition switch), the soak timer measures an elapsed time from the off-time of the ignition switch. When the soak timer detects that the elapsed time from the off-time of the ignition switch reaches a setting time, the soak timer transmits an activation signal to the control portion. Then, the control portion executes a task (e.g., a diagnosis processing of an evaporation purge system) to be performed during an off-period of the ignition switch.
If the soak timer cannot measure time with accuracy, there is no credibility to the measured time. Thus, it is necessary that the control portion detects the presence or absence of abnormality of the soak timer and gives the reliability to the measured time of the soak timer on condition that the soak timer operates normally.
Generally, in a conventional abnormality diagnosis, a measured time of a counter provided in a control portion and a measured time of a soak timer are compared in a time from when the ignition switch is turned off to when a power switch (main relay) is turned off, and it is determined that the soak timer operates normally on condition that a difference of the measured times is within a predetermined time.
JP-A-2002-155798 discloses a technique in which an internal timer of a CPU starts when an ignition of an internal combustion engine is switched on, and time information generated by the internal timer and time information supplied from an external timer are compared so as to check a function ability of the timer.
However, the technique disclosed in JP-A-2002-155798 is on assumption that the internal timer of the CPU is supplied with power. Thus, the technique is suitable only in a case where the diagnosis is completed in the time from when the ignition switch is turned off to when the power switch is turned off and it is difficult to determine whether the soak timer measures time with accuracy when the power switch is in an off-state. Thus, in a case where the soak timer operates abnormally and cannot detect time with accuracy while the power switch is in the off-state, even if the soak timer turns on the power switch at an inappropriate time to activate the control portion, the abnormality may be missed.